Stents are tubular support structures that may be implanted into body vessels to treat blockages, occlusions, narrowing ailments and other problems that may restrict flow through the vessel. Numerous vessels throughout the vascular system, including peripheral arteries, such as the carotid, brachial, renal, iliac and femoral arteries, and other vessels, may benefit from treatment by a stent.
Stents generally comprise a framework of interconnected struts that allow the stent to be collapsed into a low profile configuration for delivery and then radially expanded at the treatment site to contact the vessel wall. Balloon-expandable stents expand in response to the inflation of a balloon, whereas self-expanding stents deploy automatically when released from a delivery device.
Self-expanding stents are often fabricated from superelastic or shape memory alloys, such as Nitinol, which can “remember” and recover a previous shape. For example, a self-expanding stent made of a shape memory alloy may be trained to have a memory of an expanded configuration which it recovers after passing through the vessel in a compressed, low profile state. In the case of Nitinol alloys, the source of the shape recovery is a phase transformation between a lower temperature phase (martensite) and a higher temperature phase (austenite) that may be driven by an increase in temperature (shape memory effect) or by the removal of an applied stress (superelastic effect).
The process of training a stent made of a Nitinol alloy to have a particular remembered shape generally includes heat setting the stent while it is constrained in the configuration of interest. For example, a laser-cut stent may be disposed about a mandrel having an outer diameter corresponding to the desired inner diameter of the expanded stent, and then heated at a temperature appropriate to “set” the desired expanded shape. To facilitate positioning the stent about the mandrel for heat setting, a tapered mandrel may be employed to provide for a gradual radial expansion of the stent. To further aid the expansion, the stent may be cooled (e.g., by spraying a coolant onto the stent) so as to transform the Nitinol alloy to the low temperature martensitic phase, which is more readily deformed than austenite. As the cooled stent is advanced over the tapered mandrel, however, portions of the strut framework can experience high stresses that may result in strut misalignments. After the stent is disposed on the larger diameter of the mandrel, a tedious process of reorienting misaligned struts may be necessary before the stent undergoes the heat setting treatment to set the expanded shape.